Release coatings are typically defined as compositions which control or eliminate the adhesion between two surfaces, in particular between the surface of a first substrate onto which the release coating is applied (the backing material) and the surface of a second substrate (the adhesive material) which is coated with adhesive. Although when brought into contact, the backing material and adhesive material may be temporarily bound together, the release coating will allow them to be easily separated again, e.g. simply by manual peeling. In addition to these release properties, release coatings should also have good cohesive strength, film-forming properties, good adhesion with their substrate (i.e. with the substrate onto which they are coated), and, when brought into contact with an adhesive material, no significant contamination or component migration into the adhesive coating.
Silicone compounds, such as organosiloxanes, are well known for their use in the production of release coatings. Other types of release agents which have been used include synthetic polymers such as polyolefins and fluorocarbons, long chain alkyl derivatives such as fatty ester synthetic waxes and waxes such as petroleum, vegetable and animal waxes. Whilst all of these materials, but particularly the silicone compounds, have been shown to have suitable release characteristics for various applications, they are not useful when trying to produce paper products which can be repulped and/or recyclable. What's more, synthetic polymers tend to be prohibitively expensive. Silicones, for instance, must be cross-linked (or “cured”) to give them suitable film-forming properties and to reduce component migration. Cross-linking of silicones is typically catalysed by platinum which is both costly and toxic, making its use undesirable. As such there was a need in the industry to develop alternatives to these petrochemical-derived compounds.
Starch and starch derivatives have been used for some time as “green” alternatives to synthetic compounds in the production of traditional coating products. They have been shown to be able to provide, amongst other things, good printability, strength and even certain water resistance properties. EP1072658B1 discloses the use of starch esters having an ester component of 2 to 8 carbon atoms and a DS of from 1.0 to 2.2 in release coatings and moisture vapor barrier coatings. Unfortunately, however, such starch esters are still prohibitively expensive to produce because of their high levels of modification. They also require the use of synthetic reactants having C2 to C8 carbon chains, such as propionic acid and butanoic acid. As such, coatings produced according to EP1072658B1 will not be entirely bio-based or sustainable.
There is therefore a clear need in the industry for improved bio-based release coatings suitable for use in the production of repulpable, recyclable paper products.